Around 470 million years ago, what is now central Sweden was covered in a shallow, ancient sea inhabited by tiny, plankton-like organisms. The placid scene would soon be scarred by one of the largest cataclysms in the last billion years, scientists reported Monday.
That's because far away, trouble was brewing. In the main asteroid belt, between Mars and Jupiter, two space rocks were about to collide. When they slammed together, the collision shattered a 200-kilometer-wide asteroid, sending fragments ricocheting through space—some of which headed right for planet Earth.
As they traveled through the inner solar system, a portion of these pulverized bits and pieces re-congealed, forming what's known as a rubble pile asteroid—a type of space object that is exactly what it sounds like. But this rocky swarm wasn't like most of the others: It had a small, orbiting companion.
And when that twosome finally plowed into the ancient Swedish sea after a 12-million-year journey, it left a distinctive double crater. Or rather, a double crater that would have been distinctive had the smaller of the two punches not remained hidden until just a few years ago.
"We are quite convinced that the two craters were formed at the same time," says Erik Sturkell of the University of Gothenburg, who presented the story Monday at the American Geophysical Union's annual meeting.